Accepted Answer

There's an explanation of the grotto names and quality here: www.woodus.com/den/games/dq9ds/grotto_names.php

There's also a calculator that will predict treasures available from a dungeon based on its name and some other information like Hero's maximum level. I don't know much about it beyond its URL, but you could use it to make predictions. www.woodus.com/den/games/dq9ds/dq9_grotto.html#Analyze_H0_Netxc_I0_R0

Other Answers

I think it has something with the grotto name. I think grottos that start with the word...bronze for example, will have lower ranked chests while grottos that start with the word sapphire will have much better ranked chests.